The invention relates to a cost-effectively produced device for static mixing and heat exchange, particularly for cooling or heating of fluids, whereby the fluids comprise for example viscous or highly viscous fluids, in particular polymers.
Static mixers are used in many areas of the manufacturing industries. A characteristic of a static mixer is, that the flowable medium to be mixed is moved across at least one stationary mixer insert. The mixer insert used typically contains built-in elements, which cause a deflection of the fluid stream or of the flowable medium, which is guided through the mixing chamber. As opposed to dynamic mixers, a static mixer is free of moving parts. Dynamic mixers include for example, agitators, so that by the stirring a mixing of flowable medium is performed. In the static mixer, the flowable medium is transported through the static mixer by generating a pressure gradient. The pressure gradient may be generated for example by the use of pumps.
A number of different static mixers are known as described for example in CH-C-642564, EP0727249 B1 or EP0646408 B1 which are suited for mixing of fluids and more specifically suited for mixing viscous fluids. The task of the static mixer can be described as to produce homogeneous flowable media, which may be further processed in downstream process steps.
Such static mixers can also be cooled or heated from the outside, for example, by a jacket. In this case, the static mixer fulfills the function of homogenizing the flowable medium and in addition fulfills the object of a heat transfer, by heating or cooling the flowable medium. Under homogenizing a levelling out of physical and/or chemical and material properties of the flowable medium is to be understood which occurs for example by mixing the medium.
The heating and/or cooling capacity in static mixers is higher as compared to empty tubes with a double jacket, since the boundary layer between the fluid and the tube wall are replaced continuously by the mixer inserts arranged in the interior of the double jacket. The tube surface available for the heat exchange of the inner wall of the double jacket is predetermined and limited accordingly. It decreases with the increasing size relative to the volume of the interior space. Therefore, the cooling capacity and/or the heating capacity of such static mixers are especially severely limited for larger throughputs and sizes, if they are used as a heat exchanger. As a work around of this problem, shell and tube heat exchangers can be used for larger throughput quantities. A plurality of mutually parallel tubes are arranged in such a shell and tube heat exchanger. A fluid to be cooled or to be heated circulates through these tubes. A heat transfer fluid flows outside the tubes, for example as described in U.S. Pat. No. 6,206,086 B1. Even in such shell and tube heat exchangers, static mixers may be incorporated into the parallel tubes so as to additionally increase the heat exchange. Such heat exchangers have the disadvantage that the fluid flow has to be distributed to the individual tubes and thereby only the individual fluid strings can be mixed over the whole cross-section but not the entire fluid flow. Especially when cooling and for fluids with higher viscosity it can't be guaranteed with the device of U.S. Pat. No. 6,206,086 B1 that a homogeneous pattern of fluid flow is observed in all tubes. This can lead to different residence times of the fluid in the tubes, to deposits of the fluid along the walls of the tubes and depending on the product also to the decomposition of the fluid. Due to such effects, the cooling power of the device is reduced.
U.S. Pat. No. 7,997,327 B2 describes a heat exchanger in which the heat transfer fluid flows inside tubes which are arranged in the fluid flow similar to static mixing elements. The tubes are arranged as a tube bundle in a mixing space which is surrounded by a jacket tube. Such devices are hereinafter referred to as bundle heat exchangers. By the way of arrangement of the tubes a substantial heat transfer surface can be housed within a relatively small apparatus volume even in larger sized apparatuses, which results in a high cooling and/or heating capacity. In addition, the fluid stream does not need to be divided, but flows as a strand through the heat exchanger. The mixing and homogenization capacity of such heat exchangers is limited, however, because the crosswise arranged tubes in the mixing chamber are not considered as an ideal arrangement for mixing purposes. The tubes have too little resistance for generating an efficient cross-flow as compared to webs. The mixing effect is further reduced by the arc shaped tube especially in the marginal region adjacent to the inner wall of the jacket tube, which leads to a poor utilization of the heat transfer surface area of the jacket tube since the boundary layers along the jacket tube are insufficiently renewed. Since the tube bundle is connected only via the top plate to the jacket tube, the allowable pressure loss of such bundle heat exchangers is also limited.
The production costs of such heat exchangers are very high compared to conventional mixers or conventional heat exchangers, such as plate heat exchangers. Due to the resulting non-ideal mixing performance for such apparatus specifically for cooling of viscous fluids residence time differences of the fluid flow passing the heat exchanger are observed. Consequently there is a risk of deposits forming in the heat exchanger. Sensitive polymers can decompose as a consequence of the residence time differences. The non-ideal mixing performance and the mutual interference of the tubes leads together with an increased tube packing density lead also to a low efficiency of heat transfer, since the boundary layers between the fluid and the tube wall are not renewed in an ideal manner.
The document EP1967806A1 describes a possible solution to improve the mixing efficiency in apparatus. Such a solution is also described in U.S. Pat. No. 7,997,327 B2. It is proposed to use oval tubes which are connected with the broad side at a right angle with respect to the flow direction. Alternatively it is proposed to use parallel, identically oriented tubes. Both variants should help to increase the resistance for the fluid and thus to achieve better cross-mixing. In the embodiments in which the tube bundle is fixed on a head plate, the same disadvantages as described in connection with U.S. Pat. No. 7,997,327 B2 arise. Especially in the marginal region close to the inner wall of the jacket tube, the mixing effect is further reduced by the arc shaped tubes, which leads to a poor utilization of the jacket tube as heat transfer surface area since the boundary layers, which extend along inner wall of the jacket tube, are replaced insufficiently. Since the tube bundle is connected only via the top plate with the jacket tube, consequently the allowable pressure loss of such bundles apparatus is also limited.
EP1967806A1 also shows embodiments in which the oval tubes are connected to the jacket tube and the heat transfer fluid is fed through a double jacket. This embodiment improves the mixing effect in the edge region of the jacket tube due to the fact that no more arc shaped tubes have to be used. It has been shown that the production of such equipment with oval tubes and a jacket tube is very complex due to the fact that the individual tubes must be precisely connected to the casing pipe. In particular gaps must be avoided on the fluid side between the tubes and the jacket tube, due to the fact that deposits form from the fluid flow, which can lead to degradation effects of the tube material. In addition the accessibility for cleaning is greatly deteriorated.
Commercially available oval tubes have relatively large external tolerances, therefore the formation of gaps can't be prevented, so that an elaborate reworking of the oval tubes is required. Existing gaps on the fluid side can be poorly removed by subsequent processes such as welding or soldering as the transitions between the tube and the jacket tube especially in tube groups which are closely arranged behind one another are not accessible for reworking.
In addition, it is technically complex to seal the heat transfer medium constantly from the flowable medium. That means, there is a risk of a contamination of the media. A leak caused by leakage is difficult to seal due to the poor accessibility of the tubes. It has been further shown that even when using oval tubes a large amount of fluid tends to flow still laterally over the tubes and a small amount of fluid is transported across the cross-section of the apparatus. The use of flat tubes which have only lateral radii, and accordingly allow a better cross-flow of the fluid, there is a risk that they do not withstand the pressure difference between the fluid chamber and the heat transfer fluid. Thus, such flat tubes would have to be made very thick-walled. In addition to the non-ideal geometry of the tube, the mixing effect is additionally limited in the devices shown, due to the fact that all tubes are aligned when viewed in the flow direction. However, it has been shown that for achieving a good mixing effect at least one second tube group should be provided that is offset, for example, 90 degrees to the first tube group.
WO2008/141472 A1 and EP1067352B1 describe heat exchangers containing tubes in which the heat transfer fluid flows in parallel to the tubes arranged in the fluid flow. These tubes are also provided with webs, which are configured as baffles, which are mounted at an angle of typically 45 degrees to the flow direction. These webs are configured to exchange the boundary layers between the central fluid flow and the flow along the tube wall continuously. Such devices are referred to as tube-web heat exchangers. Even in such apparatus, a relatively large heat exchange surface area can be accommodated in a relatively small apparatus volume, depending on the packing density of the tubes. A problem with such devices is that with increasing tube density the mixing effect significantly deteriorates since the tubes arranged in parallel to the fluid flow disrupt the cross-flow of the fluid and therefore the mixing performance decreases. The cost of such equipment is very high compared to conventional mixers or conventional heat exchangers, such as plate heat exchangers. Due to the non-ideal mixing performance for such apparatus specifically regarding the cooling of viscous fluids differences in residence time distribution result and there is the danger of deposits. Sensitive polymers can decompose in such apparatus. The non-ideal mixing performance and the mutual interference of the tubes leads to increased tube packing density also results in a relatively poor heat transfer efficiency, because the interfaces between the fluid and the tube wall are not renewed in an ideal manner.
In document DE 689 05 806 T2 it is described a way to overcome these drawbacks. However, the tubes with a circular cross section shown in this document have the inherent disadvantage that the mixing performance is not optimal due to the small resistance to the flow of the tubes, which are arranged in this flow. Therefore, the solution shown in DE 689 05 806 T2 has proven to achieve a heat transfer from the tubes to the flow inside the tube. However, due to the geometric restriction of the cylinder geometry only a limited mixing performance is observed that doesn't encompass the entire cross section of the mixing chamber, which is named in the document as conduit. EP 1 123 730 A2 discloses a static mixer, which includes tubes as mixing elements. The tubes are arranged in lattices that are rotated around the center axis of the mixing element. There are three or four lattices used, which are arranged in an angle of 120° or 90° to each other.
In the document EP 0 967 004 A1 a static mixer is proposed as well, which is equipped with channels for a fluid heat medium. This static mixer is disposed with serrated plate elements arranged in the direction of flow, which lie crosswise onto each other. The plate elements are referred to as webs. These webs extend over the entire width of the mixing element. The webs are constructed as thick-walled elements, as they contain a channel leading transversely to the main direction of flow through each of the webs. The serrated webs cause a slight and only local cross-mixing, which does not comprise the entire cross section of the mixing chamber. The fluid flow incident on the web is divided by each web into two lateral main part streams flowing laterally past the web and at least one auxiliary part stream of the fluid flow, which is deflected from the serrated plate element from a spike peak extending into the fluid to the nearest valley of the serrated plate element. The auxiliary part stream is deflected from each of the spike peaks, so that a partial deflection of the auxiliary part stream is obtained. However, this deflection remains limited to the small auxiliary part stream and only to a part of the cross section of the mixer, as each web contains a plurality of tines. Therefore, the portion of transverse mixing in the mixer shown in EP 0967004 A1 is small.
A variant of such a static mixer is shown in EP 1384502 A1. As described in EP 0967004 A1 the channels for a heat transfer fluid extend substantially transverse to the main flow direction. The channels of EP 1384502 A1 run inside of finned tubes. The ribs may, for example, project into the fluid flow in a star-shaped manner. These ribs cause a slight deflection or transverse displacement of the fluid flow, which remains restricted to a local environment of the ribs. Since the ribs are not flowed through by a heat transfer fluid, their use as a heat exchange surface is limited. On the other hand they require a relatively large amount of space. Therefore a more dense packing of tubes which can be flowed through by the heat transfer fluid can't be realized, and accordingly the obtainable heat transfer surface area is reduced.
Under local mixing a cross-mixing in the immediate vicinity of the finned tube is to be understood, i.e. an environment that is limited in its size to twice the tube diameter and occurs at most to the end of the ribs. A plurality of tubes is arranged side by side transversely to the flow direction. That is, for two tubes at most half of the fluid flowing to the tubes as auxiliary part stream is guided along the edges of the ribs and can thereby cause a transverse flow. Here, too, a plurality of tubes is arranged side by side transversely to the main direction of flow. The transverse mixing occurring only over a part of the mixer cross-section can also lead to the formation of locally different thermal profiles and concentration profiles, which can have the consequence that with this mixer no homogeneous mixture is obtainable. A homogeneous mixture can only be ensured if a part of the fluid is transversely mixed over a large part of the whole cross-section.
It is mentioned in particular in connection with an embodiment, that a defined gap to the housing is desired, such that a complete flow around the web tubes is possible. In this embodiment a plurality of rows of web tubes are arranged in the flow direction one behind the other. This means that the partial flow which flows towards a web tube, although divided by the web tube, is joined downstream of the web tube again, so that a mixing takes place only within the part stream. Adjacent part streams which are shared by adjacent web tubes are not influenced by this mixing, thus the mixing occurs only locally.
There are mixers which have fittings which permit thorough mixing of the total fluid flow over the entire cross section of the mixer, such as the static mixer shown in U.S. Pat. No. 4,466,741. This mixer consists of successively arranged mixing elements. Each of these mixing elements contains the installations, which divide the flow as a cutting element and redirect the streams. For this purpose, the deflecting baffles on which cause a partial flow of the right half of the mixer is deviated to the lower half of the mixer. Adjacent mixing elements are arranged such that the partial streams are continually subdivided and deflected. However, these inserts are thin-walled channels can't be provided in such installations. The reason for this is that thin-walled webs have a lower pressure drop for the same resulting mixing action. Thus, mixers are designed with sleeve, so that the thin-wall fittings are fixedly connected to the sleeve in order to ensure the necessary dimensional stability.
A further example of such a static mixer is to be found in WO2007/113627 A1. In this document, thin walled inserts are shown as well. These inserts would not be suitable for channels which extend in the interior of these inserts. In FIG. 5 an embodiment with cross-wise arranged webs is shown, which is executed as a welded configuration. Until this point in time it was uncommon to manufacture the crossing webs in a thin-walled configuration as a monolithical part. The document US2004/0114461 A1 shows, that for realizing a monolithical mixing element thick-walled webs had to be provided. The end portions of the web elements are according to this embodiment not connected to the inner wall of the mixer. The mixer element is fixed to the mixer wall by annular segments. These annular segments form the carrier element for the web elements. The end portions of the web elements extend freely into the flow, they are not fixed to the mixer inner wall. The thick walled configuration is therefore necessary for the structural stability of the web elements, in particular if the mixer is used for highly viscous fluids. No hollow spaces such as channels should be foreseen inside the web elements for the reason of the strength of the materials. Due to the fact that the cladding element and the end portions of the web are not connected to each other the heat exchanging fluid can't enter the cladding element through the end portions of the web element, it would not be considered as an advantage to foresee channels in these thick walled webs. If a channel extends in the interior of the web elements, the heat exchange fluid in the web elements will not be able to circulate and therefore no heat supply or heat discharge may occur. For this reason the device of US2004/0114461 A1 would not be suitable as a heat exchanger.
It is an object of the invention to provide a device for mixing fluid media optimally and cool them efficiently at the same time or heat them. In addition, the device should be configured such that it can withstand high fluid pressures and is suitable for processing of viscous or highly viscous fluids. In addition, the device should not have any gaps on the side facing the flowable medium, which may lead to deposits. In addition the device should be manufactured by an economic manufacturing method.